Medical devices may monitor physiological activity of a patient. This physiological activity may be tracked to detect one or more physiological events, identify one or more symptoms, diagnose one or more medical conditions, and/or deliver therapy appropriate for the physiological activity. For example, an implantable cardiac device (e.g., a cardiac pacemaker or implantable cardioverter defibrillator (ICD)) may sense electrical activity (e.g., a sensed electrocardiogram (ECG) signal) of a heart of the patient and determine a heart rate of the patient based on the sensed electrical activity. If the cardiac device determines that the heart rate is indicative of an arrhythmia, the cardiac device may deliver pacing pulses or a shock to the heart to correct the arrhythmia.